Microphone.



No. 771,135. PATENTED SEPT. 27, 1904.

E. B. FAHNESTOGK.

MICROPHONE.

APPLICATION IILED OUT. 25, 1902.

NO MODEL. J? 76 J3 UNITED STATES Patented September 27, 1904.

PATENT OFFICE.

MICROPHONE.

SPECIFICATION forming part of Letters Patent No. 771,135, dated September 2'7, 1904..

Application filed October 25, 1902.

To all whom it may concern:

Be it known that I, ERNEST B. FAHNESTOCK, a citizen of the United States, residing at New York city, county and State of New York, haveinvented certain new and useful Improvements in Microphones, of which the following is a specification.

My invention relates to microphones of the type employed for transmitting sound; and it consists of a construction in which the sounddiaphragm is so arranged that it will not be subjected to mechanical-strains and, further, will only beacted upon by the air-waves imping ing upon its external surface.

In experimenting with microphones I have observed that if the diaphragm is secured at its periphery the transmitting value of the sound-waves impinging upon its external surfacewill be decreased by the normal resistance against inward movement presented by the material of which the diaphragm is made, and, further, that if a diaphragm is mounted at its center and free to vibrate at its periphery the transmitting value of the air-waves impinging upon its outer surface will be decreased by the influence of the same air-waves acting in the opposite direction upon its inner surface, or, in other words, the mechanical resistance presented by the material of the diaphragm against motion or the influence of the sound-waves acting upon the back of the diaphragm will both serve to dampen the movement of the diaphragm.

The object of my invention, therefore, may be said to be to improve the transmitting quality of a microphone by a const uction which will remove the dampening effect mentioned.

The accompanying drawings will serve to illustrate my invention.

Figure 1 is a section taken on the line X X of Fig. 2. Fig. 2 is a view in perspective looking from the right. Fig. 3 is a similar view with the diaphragm removed. Figs. 1 and 5 are sections corresponding to Fig. 1, showing modifications of my invention. Figs. 6 and 7 are perspective views looking from the left of the two forms of diaphragm shown in Figs. 1 and 4:.

My improved microphone may make use of Serial No. 128,745. (No model.)

one or more than one diaphragm. I will therefore for clearness describe a microphone having but a single diaphragm.

'In the drawings, 5 indicates the supporting-body of the microphone, which may be made of metal or other suitable material and given any required shape. Situated in the center of the body 5 is a carbon-chamber 6,

adapted to contain granular carbon 7. Located within this chamber are electrodes 8 9. I prefer to use two electrodes. One, however, may be employed. These electrodes may be formed of carbon or metal, as desired. Arranged over and forming a cover for the carbon chamber is a flexible diaphragm 10, formed of mica or other suitable material. In Fig. 5 one such diaphragm is shown; in Fig. 1, two. The diaphragm 10 is secured to the casing 5 by means of a metallic ring 11 and screws 12. Any other means may be employed for securing the diaphragm. An electrode 8, Fig. 5, is connected to the diaphragm 1O (8 9, Figs. 1 and 4:) through the instrumentality of a stud 13, which passes through an opening in the diaphragm and has arranged over it external to the diaphragm a washer 14. and nut 15.

Mounted centrally upon the stud l3 and over the nut 14 is a main diaphragm 17, which is much larger than the diaphragm 10. This diaphragm is secured upon the stud 13-by the nut 18. In the construction shown in Figs. 1, 2, 3, 5, and 6 the diaphragm 17 has its periphery turned inward at right angles to its surface to form a flange 19. The diaphragm 17 when in position is located in a chamber 20, formed in the face or faces of the body 5, and the outer edge of the flange 19 is arranged concentric with the inner wall of the chamber 20. In order to prevent the entrance of airwaves behind the diaphragm, I introduce between the flange 19 and the inner wall of the circular chamber 20 a thin body of an elastic cement21. Ipreferably usea rubber-cement.

Any cement will serve the purpose which will not rigidly secure the periphery of the diaphragm to the body of the microphone. In place of using a rubber-cement, as described, I may arrange to have the flange 19, in a desk instrument adapted to be used in a horizontal position, dip into a body of mercury 22, held in an annular channel 23, formed in the body of the microphone 5, or instead of using a diaphragm provided with a flange I may use a flat diaphragm without a flange, as shown in Fig. 4c, and to prevent the air-waves from entering behind thisdiaphragm I connect to its periphery an annulus of flexible material 24:. Such annulus may be made of thin sheetrubber, cloth, or other flexible material impervious to air. This annulus is cemented at its inner periphery to the periphery of the diaphragm and secured at its outer periphery through a ring 25 and screws 26 in the circular chamber 20 in the body of the microphone. The construction of this form of diaphragm is shown in Fig. 7.

27 28 are conductors connected,respectively, with the electrodes 8 9. These conductors are carried out of the sides of the supportingbody 5 and thence through slots 29 in the sides of the supporting-body upward to the binding-screws 80 on the top of the body. This I iind to be a convenient way for establishing electrical connection with the interior of the microphone. Other means may be employed, however.

I wish it understood that I do not in any Wise limit myself to the particular constructions of the diaphragm or to the means employed for preventing access of air-waves under the diaphragm, as described. I have shown in the drawings two forms of diaphragm and three constructions by means of which the airwaves can be prevented from finding access behind the diaphragm, and I wish to have it understood that I consider any means for effecting this result without exerting mechanical tension upon the diaphragm as within the contemplation of my invention.

It will be seen upon reference to Figs. 1, 4, and 5 that the diaphragm 17, by reason of its central mounting and freedom from mechanical restraint at its periphery, is free to vibrate as a whole in parallel planes and is not subjected to the dampening effect of a rigid connection at its periphery or the action of the air-waves behind the diaphragm.

Having thus described my invention, 1 claim- 1. In a sound-transmitting microphone, the combination of a supporting -body, a diaphragm mounted at its center and having a flanged periphery, and a flexible body interposed between said supporting-body and the flange of the diaphragm.

2. In a sound-transmitting microphone, the combination of a support, a diaphragm having a flanged periphery, and a flexible body interposed between said support and the flange of the diaphragm.

3. In a sound-transmitting microphone, the combination of a supporting-body having a recessed face, a diaphragm having a flanged periphery, and a flexible uniting material interposed between the supporting-body and the flange of the diaphragm.

4. A sound-transmitting microphone, em-

bodying in its construction, a supportingbody, said supporting-body having a centrally-disposed carbon-chamber-and provided with a groove in its outer face, electrodes in said carbon-chamber,/carbon between said electrodes, a flexible cover for the carbonchamber, a diaphragm mounted upon the flexible cover and free to vibrate at its periphery, and means interposed between the periphery of said diaphragm arid said groove in said supporting-body for preventing airwaves from entering behind the diaphragm. In testimony whereof I aflix my signature in the presence of tWo witnesses.

ERNEST B. FAHNESTOCK. WVitnesses:

J. E. PEARSON, 1 FRANK OOONNORa 

